It was less than two months ago that rumors of a Microsoft competitor to Slack, the messaging app popular in tech and other creative industries, began leaking to the public.

Microsoft Teams has been quick to market, with Microsoft CEO Satya Nadella announcing Wednesday that commercial customers in 181 countries are now previewing the app. And now you can download Microsoft Teams yourself, with desktop versions for Windows and Mac available on the Microsoft website. Mobile versions for iPhone, Android and Windows are also available.

Included with Office 365 Business and Enterprise, the service offers both text and Skype-enabled video chat, facilitating communication among users whether they’re sitting near each other or working remotely.

The Office 365 and Skype integration make Microsoft Teams a winner among large companies that want to improve their internal communication. But smaller, more nimble businesses may appreciate that Slack enables communication with anyone–freelancers, vendors and others not part of their organization. They also may want a service that at its most basic level is free, without having to purchase Office 365.

As The Verge’s Tom Warren points out, Microsoft has open-sourced the Teams APIs and is encouraging developers to build extensions. How developers choose to innovate on each platform may determine their usefulness in the future.

Slack has been a darling of the tech industry, widely adopted and winning accolades for its efforts to increase diversity among its employees. Don’t count it out yet.

But as a defensive open letter published by Slack in the New York Times shows, the plucky startup is clearly worried about competition from the Microsoft behemoth.

Slack or Microsoft Teams? Give us your take in the comments.